1. Field of the Invention
The present invention relates to a mobile communications system which enables two-way communications with a base station.
2. Description of the Prior Art
There have been proposed several mobile communications systems which can perform two-way communication between a mobile terminal and a base station.
In a paging system disclosed in Japanese Patent Application Laid-Open No. 62-269524, when receiving a paging signal from a radio base station, a mobile terminal transmits a reply signal thereto back to the base station and the replay signal is transferred to a control station of the system.
In another paging system disclosed in Japanese Patent Application Laid-Open No. 3-38122, when receiving a paging signal from a base station, the called pager transmits a predetermined recognition signal back to the base station. and the base station, when receiving the recognition signal, sends an acknowledgement to the caller.
A selective call receiver disclosed in Japanese Patent Application Laid-Open No. 63-174433 is provided with a function such that, when receiving a message including a replay request from a source terminal, the called terminal calls back to the source terminal.
However, in the above paging systems, since the mobile terminal receives a paging signal from the wide area selective calling system and then transmits a reply signal to the base station, such a mobile terminal is usable only within the service area of the wide area selective calling system, so that the conventional two-way communications system cannot achieve sufficient system performance and reliability.
Further, mobile terminals carried by users are battery-powered. Therefore, it is desirable that their power consumption be minimized. There has been proposed a radio telephone terminal that is intended for such reduction in power consumption in Japanese Patent Application Laid-Open No. 7-303077. This radio telephone terminal is equipped with a detector for detecting the remaining amount of battery power. If the battery remaining power is greater than or equal to a predetermined amount, location registration is performed every time the telephone terminal moves to an adjacent communicable area (cell). If the battery remaining power is smaller than the predetermined amount, the user is merely informed of non-effectiveness of location registration. Further, the time interval of reception of control signals from a base station and that of location registration are varied in accordance with the battery remaining power, to reduce the battery consumption and in turn elongate the usable time.
However, a transmission operation for location registration requires a large amount of power because of driving of a high-frequency amplifier. In the conventional radio communication terminal as described above, because it is necessary to perform, on a regular basis, transmission and reception operations for location registration with the base station, the battery is consumed at a high rate and the usable time cannot be made sufficiently long.